Egg and time limitation mediate an egg protection strategy

Abstract

The number of mature eggs remaining in the ovaries and the time left for oviposition determine the reproductive decisions of the hyperdiverse guild of insects that require discrete and potentially limiting resources for oviposition (such as seeds, fruits or other insects). A female may run out of eggs before all available oviposition sites are used (egg limitation), or die before using all of her eggs (time limitation). Females are predicted to change clutch size depending on whether eggs or time is the limiting resource. We extend this framework and ask whether the same constraints influence a strategy in which females modify eggs into protective shields. In response to egg parasitism cues, female seed beetles (Mimosestes amicus) lay eggs in vertical groups of 2-4, modifying the top 1-3 eggs into shields in order to protect the bottom egg from attack by parasitoids. We made contrasting predictions of how egg and time limitation would influence egg size and the incidence and level of egg protection. By varying access to seed pods, we manipulated the number of remaining eggs a female had at the time she received a parasitism cue. Although egg size was not affected, our results confirm that egg-limited females protected fewer eggs and time-limited females protected more eggs. Female body size explained the number of eggs in a stack rather than host deprivation or the timing of parasitoid exposure. Our results clearly show that host availability relative to female age influences the incidence of egg protection in M. amicus. Furthermore, our study represents a novel use of life history theory to explain patterns in an unusual but compelling defensive behaviour.

Invasive Egg Predators and Food Availability Interactively Affect Maternal Investment in Egg Chemical Defense

“…The total reproductive capacity of females is finite and therefore a reduction in offspring investment at risky or poor egg laying sites can enable them to increase their investment at more favorable sites, thus maximizing female fitness (Rosenheim, 1999). This does not necessarily involve the cessation of egg laying at risky sites, but a reduction, for example in offspring number (Guo et al, 2014), or in a costly aspect of offspring phenotype (Deas and Hunter, 2014). However, if A. bipunctata females were employing this strategy in the face of predation risk a reduction in egg alkaloid levels between predator treatments when aphids were present as well as absent, and in other markers of investment, such as egg size or number, may reasonably have been expected, but this was not seen.…”

Host patch quality increases parasitoid locomotor activity despite risk of egg limitation

“…To maximise reproductive potential, female insects must balance the risks of becoming egg or time‐limited (Deas & Hunter, 2014; Dieckhoff et al, 2014; Minkenberg et al, 1992; Rosenheim, 1999; Rosenheim et al, 2008). Individuals that are egg‐limited exhaust their egg supply before death, while time‐limited individuals fail to fully exhaust their egg supply before death (Deas & Hunter, 2014).…”

“…To maximise reproductive potential, female insects must balance the risks of becoming egg or time‐limited (Deas & Hunter, 2014; Dieckhoff et al, 2014; Minkenberg et al, 1992; Rosenheim, 1999; Rosenheim et al, 2008). Individuals that are egg‐limited exhaust their egg supply before death, while time‐limited individuals fail to fully exhaust their egg supply before death (Deas & Hunter, 2014). Proovigenic species, which are unable to mature new eggs after emerging as adults, are often thought to be more susceptible to egg limitation, while strongly synovigenic species, which mature eggs throughout adulthood, are thought to be more time‐limited (Jervis et al, 2001).…”

Multigenerational experimental simulation of climate change on an economically important insect pest

“…Resource allocation theory assumes that organisms have a limited amount of resources, which will be traded off among life history traits (Boggs, 2009; Deas & Hunter, 2014; Parker & Courtney, 1984; Pianka, 1981; Roff, 2002). For the case of females, a large proportion of their resources must be allocated to producing successful offspring.…”